1. Field of Disclosure
The invention relates to drill stem components used for the rotary drilling of oil or gas fields. In particular, the invention is applicable to components used in a drill stem such as drill pipes or heavy weight drill pipes, for example.
2. Description of Related Art
Rotary drill pipes connected together to form drill strings and associated with other components of the drill stem (drill collars, stabilizers, etc) can be used to produce deviated bores, i.e. bores wherein the inclination to the vertical or the horizontal direction can be varied during drilling. Deviated bores can currently reach depths of the order of 2 to 8 km and horizontal distances of the order of 2 to 15 km.
However, in the case of deviated holes, a number of problems arise which are directly linked to the variation in the inclination of the bore.
Firstly, on sections of drill strings that are almost horizontal, frictional torques may reach very high values during drilling under the effect of the weight of the components employed on those sections. This results in premature wear of the components employed on those sections.
Next, because the hole is no longer rectilinear, it is much more difficult for mud loaded with debris derived from excavating the rock to ascend when it is not straight. This results in poor cleaning of the hole and an increase both in the coefficients of friction of the pipes of the drill string inside the drilled hole and the contact surfaces between the pipes and the walls of the hole.
Finally, because the trajectory followed by the drill string is no longer rectilinear, it appears that the distribution of vibrations along such strings is no longer homogeneous. For this reason, bending stress concentrations that are over the admissible limits risk damaging the drill strings in certain regions.
In order to overcome these disadvantages, the prior art has proposed a variety of arrangements.
Thus, document FR 2 851 608 describes a drill pipe provided with a bearing zone having a hard coating so that at that region, the contact surface with the walls of the hole is wear-resistant. Furthermore, activation zones which are helical in shape can accelerate the ascent of drilling fluid and debris derived from drilling.
Similarly, document FR 2 835 014 proposes drill pipe profiles with depressions and projections which are arranged to facilitate the ascent of drilling debris.
Clearly, those solutions have produced very satisfactory results. However, the current solutions require that the drilling components be machined in order to obtain the activation zones, and the bearing zones of said components have to be treated in order to obtain a wear-resistant coating. More generally, adding such functions to the drilling components has a huge impact on the manufacture of such components.
Document WO-2005/93204 proposes a device that can be fixed on a drilling component in a removable manner and has functional zones which can facilitate the movement of drilling mud and the ascent of debris as well as progress of the component during drilling. The device is constituted by two half-shells connected together via a pivotal connection; the device docks with the drilling component by means of a clamping system which positions the half-shells flush against each other.
However, this solution suffers from the disadvantage of rendering the drilling component fragile. The fact that the device surrounds the component generates a zone with a high concentration of stresses. The effect of this is that the drilling component can break in service, or it can become detached from the device carrying the functional zones.
The invention proposes a drilling component comprising functional zones that are even more resistant in operation.
More precisely, a rotary drill stem component for exploration of a hydrocarbon well with drilling mud in movement around said component from the bottom of the well towards the surface comprises a central tubular element having an axis of revolution and extended on either side respectively by a first and a second tool joints each respectively and successively comprising a first, cylindrical, portion connected to the tubular element, a second, tapered, portion, a third, cylindrical, portion with a radius R which is greater than that of the tubular element, and a threaded end which can connect the component to another component. The component comprises a shell with one or more functional zones provided on its circumferential surface. The shell is mounted on one of the tool joints in a cohesive and removable manner.